Then I connect the keg to co2 and draw off a few ounces into a glass. I drink that beer so it cannot be called waste.
Warm, flat and yeasty, but not wasted!
If you're doing a gravity transfer with the gas line returning to the fermenter, flow will stop when the level in the line equalizes, and you'll only leave a small amount in the line.
I transfer pushing with CO2. I used to place the line out of the gas post in a bucket of iodophor, and it would give an audible signal -- rapid bubbling -- that beer was flowing. When the bubbling slows radically, stop those other chores in the brewery instantly, the keg's nearly filled. Thanks to forum member macbrews for the idea.
My current adaptation is to put a picnic tap on the gas post. Bleed pressure periodically as needed to keep flow going -- sort of a poor man's counter pressure fill. Then when beer finally exits the port, you needn't spill a drop. Then a picnic tap over on the liquid side to draw off another pint, and no "waste" if you drink it, or at least cook with it. But like BrewBama, I consider it part of my planned losses. It's minimal though.